The present invention relates to a semiconductor thin film substrate, a semiconductor device, a semiconductor device manufacturing method and an electronic apparatus, and more particularly, to the technology for manufacturing transistors (for example, a thin film transistor (TFT)) using polycrystalline films (polycrystalline semiconductor thin films), a semiconductor thin film substrate for manufacturing the thin film transistor, and the technology which is effectively applicable to manufacturing processes for electronic apparatus such as a liquid crystal display device, an information processing apparatus and so on that incorporate the thin film transistors.
Thin film transistors so far used in conventional image display devices and so on are formed using polycrystalline silicon which is fabricated on an insulating base made of glass, quartz or the like by a recrystallization method such as excimer laser anneal or the like, using amorphous silicon or micro-crystalline silicon formed by a plasma CVD method or the like as a precursor.
A conventional method of manufacturing a polycrystalline semiconductor thin film (polycrystalline silicon thin film) and a thin film transistor will be described below with reference to FIGS. 1a-1d, 2, 3, 4a-4c, and 5a-5c. 
As illustrated in FIG. 1a, a silicon oxide film (SiO2 film) 102 and an amorphous silicon thin film 103 are sequentially formed on an insulating base 101, for example, a glass base 101. Next, as illustrated in FIG. 1b, the surface of the amorphous silicon thin film 103 is irradiated with excimer laser light 105 which has light flux of rectangular or elongated cross-section. As indicated by an arrow 106, the laser light 105 is moved (scanned) to anneal the overall surface of the amorphous silicon thin film 103 with the excimer laser light 105. The amorphous silicon thin film 103 changes from an amorphous structure to a polycrystalline silicon thin film 104 by this annealing through a melt/solidification process, as illustrated in FIG. 1c. 
The foregoing process is referred to as an excimer laser annealing process (excimer laser crystallization), and is used for fabricating a high quality polycrystalline thin film on a base made of a low melting point material such as glass. The excimer laser annealing process is described in detail, for example, in xe2x80x9c1996 Society for Information Display International Symposium Digest of Technical Papers,xe2x80x9d pp. 17-20, and xe2x80x9cIEEE Transactions on Electron Devices,xe2x80x9d vol. 43, no. 9, 1996, pp. 1454-1457, and so on.
FIG. 1d is a schematic diagram illustrating a TFT which has been formed using the aforementioned polycrystalline silicon thin film 104. In the polycrystalline silicon thin film 104, semiconductor regions 110, 111 are formed by diffusing a predetermined impurity element. These semiconductor regions 110, 111 constitute a source region and a drain region of a field effect transistor. Also, a gate insulating film 112 made of SiO2 is provided on the surface of the polycrystalline silicon thin film 104 between the semiconductor regions 110, 111, and a gate electrode 113 is provided on the gate insulating film 112. In this structure, a source-to-drain current can be controlled by a voltage applied at the gate electrode 113. For example, the gate has a length of 4 xcexcm and a width of 4 xcexcm.
FIG. 2 is a graph related to the dependency of the silicon crystal grain diameter on an irradiated laser energy density in the conventional excimer laser crystallization. In this example, an amorphous silicon thin film 103 formed on an insulating base 101, for example, a glass base 101, has a thickness of 100 nm, and is crystallized by XeCl excimer laser-based annealing (at wavelength of 308 nm). As can be seen from the graph, the amorphous silicon thin film is not crystallized at a laser energy density below 100 mJ/cm2 since the thin film is not melted with such energy. However, the thin film is melted from its surface as the energy density exceeds 100 mJ/cm2, resulting in crystal nuclei produced on a solid-liquid interface of the amorphous silicon thin film 103 and resulting formation of crystal grains (for example, crystal grains 104a).
As the laser energy density is increased, the amorphous thin film is melted deeper. As a result, larger crystal grains are produced (for example, crystal grains 4b). The production of crystal nucleus from a solid-liquid interface in this way is referred to as xe2x80x9cinhomogeneous nucleation.xe2x80x9d In FIG. 2, Ec indicates a laser energy density at which the solid-liquid interface reaches the insulating base 101. As the laser energy density exceeds Ec, the overall amorphous thin film is melted, and enters into a supercooling state. As a result, crystal nuclei are produced within the thin film at random to form micro-crystals 104c of diameters equal to or less than 0.05 xcexcm. Such production of crystal nuclei is referred to as xe2x80x9chomogeneous nucleation.xe2x80x9d
For fabricating a polycrystalline silicon thin film transistor (TFT) having satisfactory characteristics, for example, a TFT exhibiting a mobility xcexc of 100 cm2/V.s, the grain diameter of silicon crystals must be 0.2 xcexcm or more. Therefore, the amorphous thin film should be crystallized with the laser energy density set at Ec. In this example, Ec is set at 230 mJ/cm2. It should be noted however that the value of the laser energy density in the prior art may vary since it depends on the nature of the amorphous silicon film (for example, an employed growth method, its film thickness, and so on), the temperature of the base, and the wavelength and pulse width of the excimer laser. Details in this respect are found, for example, in xe2x80x9cApplied Physics Letters,xe2x80x9d vol. 63, no. 14, 1993, pp. 1969-1971, and so on.
FIG. 3 is a schematic plan view showing a positional relationship between semiconductor regions 110, 111 and a gate electrode 113 of a TFT. A channel is formed between the semiconductor regions 110, 111, and the length of the channel is equal to the gate length. The channel length may be, for example, 4 xcexcm. Also, an average crystal grain diameter (crystal diameter 104b) of crystals comprising the polycrystalline silicon thin film is 0.25 xcexcm.
Therefore, it is readily estimated that larger crystal grains are desirably formed in the channel for improving the characteristics of the TFT (increasing the carrier mobility to achieve a faster operation).
Thus, as a technique for forming position-controlled large crystal grains, a method of controlling a laser intensity distribution has been proposed. FIGS. 4a to 4c illustrate the formation of a gate insulating film (SiO2 film) 112, which is patterned to define a region in which large crystal grains should be formed, on a similar structure illustrated in FIG. 1a. As an excimer laser is irradiated, the temperature at a region of an amorphous silicon thin film beneath the gate insulating film 112 becomes higher than the remaining region, so that a resulting temperature distribution is as indicated by a temperature curve 114 in FIG. 4b. In this event, the crystallization initiates from ends of the gate insulating film 112 to form large crystal grains 121a due to a strong (large) temperature slope. Also, in this event, the crystal grains produced from both ends of the gate insulating film 112 grow to collide with one another in a region beneath the gate insulating film 112, resulting in formation of a crystal grain boundary 122, as illustrated in FIG. 4c. In regions other than the region beneath the gate insulating film 112, since the temperature slope is weak (small), a polycrystalline silicon thin film 4 is formed with crystal grains having smaller grain diameters than the crystal grains in the region beneath the gate insulating film 112.
As a technique for forming position-controlled large crystal grains, a method of irradiating excimer laser using a mask 123 is known, as illustrated in FIGS. 5a to 5c (Japanese Journal of Applied Physics, vol. 37, 1998, pp. 5474-5479).
With this technique, the amorphous silicon thin film 103 exhibits a temperature distribution as indicated by a temperature curve 114xe2x80x2 shown in FIG. 5b, wherein the temperature is higher in a region which is not covered with the mask 123. Therefore, as illustrated in FIG. 5c, the crystallization advances from an amorphous silicon thin film 103 corresponding to the end of the mask 123 to a region over which the mask 123 does not exist, and larger crystal grains 121a are formed in this region.
Thin film transistors (TFT) formed using low-temperature polycrystalline silicon (polysilicon) thin films can constitute CMOS (Complementary Metal Oxide Semiconductor) transistors, so that they can be used not only as switching devices for pixels in a liquid crystal display but also as peripheral circuit elements such as a shift register, an AD converter and so on. This is because a low temperature polysilicon thin film (low temperature polysilicon film) formed by excimer laser based crystallization is composed of large crystal grains which exhibit high crystallinity. However, for realizing a system-on-panel (a predetermined electronic apparatus implemented by mounting a plurality of transistors and so on on a single substrate) having high performance and high reliability, it is necessary to develop techniques which accomplish the following targets.
(1) An increased process margin for the excimer, laser energy density.
(2) Crystallization of larger crystal grains having crystal grain diameters of 0.2 xcexcm or more. Here, the crystal grain diameter is measured in the following procedure.
(a) Determine a region under measurement in a surface area of a polycrystalline semiconductor thin film.
(b) The area of the region under measurement is 1 xcexcm2.
(c) Take an electron micrograph of the surface of the region under measurement.
(d) Crystal grains, each of which is entirely included in the region under measurement of the electron microscopic photograph, i.e., crystal grains, each of which is entirely included on the surface of the region under measurement, are regarded as crystal grains under measurement, and the number of these crystal grains is counted. Also, the total area of the crystal grains under measurement, i.e., the total area of the crystal grains under measurement on the top surface of the region under measurement is measured from the electron micrograph.
(e) The total area of the crystal grains under measurement is divided by the number of the crystal grains under measurement to derive an average area S of the crystal grains under measurement.
(f) Assuming that the crystal grains under measurement on the top surface of the region under measurement is circular in shape, the average area S is substituted into an equation 2{square root over ((S/xcfx80))} to calculate the diameters of the crystal grains.
(3) Control of the position of a crystal grain boundary.
(4) Planarization of a polycrystalline film.
(5) Development of a self-aligned TFT.
(6) Development of a TFT in LDD (Lightly-Doped Drain) structure.
On the other hand, a review on the prior art in view of the foregoing targets of development reveals that a number of problems are found as follows.
As can be seen from FIG. 2, a laser energy region for producing larger crystal grains extends approximately from 10 to 20 mJ/cm2. However, since the existing excimer laser has an output stability of xc2x110 to 25 J/cm2 at most, a margin for the excimer laser energy density is thought to be extremely small.
Also, Ec in FIG. 2 depends on the thickness of an amorphous silicon thin film. When a change in thickness is 10% or more, a polycrystalline film changed by laser irradiation is a mixture of large crystal grains and small crystal grains. The crystallization of crystal grains having diameters of 0.2 xcexcm or more is difficult to accomplish.
Also, the diameters of resulting crystal grains vary due to a difference in the temperature slope between a region fully irradiated with laser and an end region irradiated less with the laser. The varying crystal grain diameters may cause deviations in density of trap state in a channel region beneath the gate electrode, and a consequent change in the threshold voltage Vth of respective transistors over xc2x1several volts and deviations in the carrier mobility xcexc spanning approximately xc2x150 cm2/V.s.
Also, the crystal grain position control technique illustrated in FIGS. 4a to 4c inevitably involves the formation of crystal grain boundaries within the channel region. When a number of crystal grain boundaries exist in the channel region of silicon beneath the gate electrode, the non-uniformity of crystal grains may cause the carrier mobility xcexc to be reduced to several cm2/V.s due to dispersion of conduction carriers or the like.
Also, as impurities are implanted into a polycrystalline region, the impurities are locally deposited on a crystal grain boundary so that the impurity concentration is difficult to control.
Further, the crystal grain position control technique illustrated in FIGS. 5a to 5c is not capable of forming a self-aligned TFT, and therefore experiences difficulties in a reduction in the size of the TFT.
Furthermore, the silicon channel region beneath the gate electrode is susceptible to the formation of proturberances in a grain boundary corner. The proturberances introduce a lower mobility due to dispersion of carriers, and resulting deviations and degradation in the performance of respective transistors. Particularly, proturberances formed in a drain end portion, if any, would cause a higher likelihood of a concentrated electric field, resulting in a deteriorated transistor due to the production of hot carriers.
It is an object of the present invention to provide a semiconductor device having a thin film transistor which exhibits high performance and high reliability, and an electronic apparatus which incorporates the semiconductor device.
It is another object of the present invention to provide a semiconductor device which has a polycrystalline thin film with crystal grains of larger diameters constituting a channel region of a thin film transistor, and an electronic apparatus which incorporates the semiconductor device.
It is another object of the present invention to provide a semiconductor device which has smaller proturberances in a corner of a crystal grain boundary in a channel region of a thin film transistor, and a polycrystalline thin film comprised of crystal grains with larger diameters as compared with other regions, and an electronic apparatus which incorporates the semiconductor device.
It is another object of the present invention to provide a semiconductor device which has proturberances reduced to 15 nm or less in a corner of a crystal grain boundary in a channel region of a thin film transistor, and a polycrystalline thin film comprised of crystal grains with diameters of 0.2 xcexcm or more, larger than other regions, and an electronic device which incorporates the semiconductor device.
It is another object of the present invention to improve the manufacturing yield of a semiconductor device (for example, a thin film transistor), reduce the manufacturing cost, and provide a high performance electronic apparatus incorporating a thin film transistor at a lower cost.
It is another object of the present invention to provide a semiconductor thin film substrate which is capable of producing crystal grains of larger diameters and reducing proturberances in a corner of a crystal grain boundary.
The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.
According to a first aspect of the present invention, a semiconductor thin film substrate includes an insulating base, a first non-crystalline thin film formed on the insulating base, a second non-crystalline thin film formed on the first thin film, and a non-crystalline semiconductor thin film formed on the second thin film, wherein the second thin film has a thermal conductivity higher than a thermal conductivity of the first thin film, and lower than a thermal conductivity of the non-crystalline semiconductor thin film. The non-crystalline semiconductor thin film may be an amorphous semiconductor thin film. Also, in this structure, a bulk material forming the second thin film has a thermal conductivity higher than the thermal conductivity of a bulk material forming the first thin film, and lower than the thermal conductivity of a bulk material forming the non-crystalline semiconductor thin film. The first thin film may be a silicon oxide film, the second thin film may be a silicon nitride film, and the non-crystalline semiconductor thin film may be a silicon film. Alternatively, in the composition of the respective films, the first thin film may be a silicon oxide film, the second thin film may be a silicon nitride film, and the non-crystalline semiconductor thin film may be a silicon germanium film. Further alternatively, the first thin film may be a silicon oxide film, the second thin film may be a silicon germanium film, and the non-crystalline semiconductor thin film may be a silicon film. The second thin film may be in contact with the first thin film, while the non-crystalline semiconductor thin film may be in contact with the second thin film.
According to another aspect of the present invention, a semiconductor device includes an insulating base, a first non-crystalline thin film formed on the insulating base, a second non-crystalline thin film formed on at least a portion of a surface of the first thin film, a polycrystalline semiconductor thin film formed on a surface of the second thin film or on the surfaces of the second thin film and the first thin film, and a field effect transistor having a channel formed of a portion of the polycrystalline semiconductor thin film, wherein the second thin film has a thermal conductivity higher than the thermal conductivity of the first thin film and lower than the thermal conductivity of the polycrystalline semiconductor thin film. The first thin film may be a silicon oxide film, the second thin film may be a silicon nitride film, and the non-crystalline semiconductor thin film may be a silicon film. Alternatively, in the composition of the respective films, the first thin film may be a silicon oxide film, the second thin film may be a silicon nitride film, and the non-crystalline semiconductor thin film may be a silicon germanium film. Further alternatively, the first thin film may be a silicon oxide film, the second thin film may be a silicon germanium film, and the non-crystalline semiconductor thin film may be a silicon film. The second thin film may be in contact with the first thin film, while the non-crystalline semiconductor thin film may be in contact with the second thin film.
Crystal grains in the polycrystalline semiconductor thin film constituting the channel may have measured diameters of 0.2 xcexcm or more, wherein the diameters of the crystal grains are measured, for example, in a region occupying an area of 1 xcexcm2 having a length of 0.5 xcexcm in upper, lower, left and right directions from the center of a surface region on the gate electrode side of the field effect transistor of the polycrystalline semiconductor thin film constituting the channel, wherein the crystal grains to be measured are selected to be crystal grains which are entirely accommodated in the region under measurement on the surface thereof, wherein assuming that the crystal grains to be measured on the surface of the region under measurement are circular, the diameter is determined by substituting an average area S of the crystal grains to be measured into an equation give by 2{square root over ((S/xcfx80))}. The average area is calculated by dividing a total area of the crystal grains to be measured on the surface of the region under measurement by the number of crystal grains to be measured.
Also, the second thin film may include end portions near the source region and the drain region of the field effect transistor, wherein the end portions are gradually reduced in thickness toward the source region and the drain region, respectively.
The foregoing aspect of the present invention may be applicable to other semiconductor devices of different structures in which an active region is formed in a portion of a polycrystalline semiconductor thin film, for example, to a bipolar transistor and so on.
According to another aspect of the present invention, a semiconductor device is manufactured in the following method. Specifically, the method of manufacturing a semiconductor device comprises the steps of forming a first non-crystalline thin film, a second non-crystalline thin film and a non-crystalline semiconductor thin film on one surface of an insulating base sequentially in laminate, irradiating the non-crystalline semiconductor thin film with laser light to crystallize the non-crystalline semiconductor thin film to form a polycrystalline semiconductor thin film, and forming an active region of a semiconductor element in the polycrystalline semiconductor thin film. The first thin film, the second thin film and the non-crystalline semiconductor thin film mutually have a relationship in terms of the thermal conductivity such that, after a laser light irradiation under the same conditions of the above-mentioned laser light irradiation, the thermal conductivity of the non-crystalline semiconductor thin film is higher than the thermal conductivity of the second thin film, and the thermal conductivity of the second thin film is higher than the thermal conductivity of the first thin film. The non-crystalline semiconductor thin film is formed such that a correlation of a laser energy density irradiated to the non-crystalline semiconductor thin film with a crystal grain diameters of the polycrystalline semiconductor thin film has a normal growth in which crystal grains grow from crystal nuclei to a predetermined size, a first critical energy density Ec at which the normal growth is maximum, a secondary growth in which the crystal grains formed by the first growth fuse with each other in a laser energy density range exceeding the first critical energy density Ec to grow to larger crystal grains, and a second critical energy density Ecxe2x80x2 at which the secondary growth is maximum. The non-crystalline semiconductor thin film is irradiated with laser light at a laser energy density higher than the first critical energy density Ec and equal to or lower than the second critical energy density Ecxe2x80x2 to form the polycrystalline semiconductor thin film.
The second thin film is disposed in a predetermined pattern on at least a portion of a region of the first thin film corresponding to the channel. The second thin film may be formed in a double-sided comb-shaped pattern having digits, each of which extends along the lengthwise direction of the channel.
A light shielding film formed in a pattern corresponding to the position of the channel of the field effect transistor may be disposed between the first thin film and the second thin film, in which case a source region and a drain region of the field effect transistor may be formed in the polycrystalline semiconductor film region such that the source and drain regions are self aligned to the light shielding film. The source region and the drain region have a lower impurity concentration in portions near the light shielding film than in portions away from the light shielding film.
According to another aspect of the present invention, an electronic apparatus incorporates a semiconductor device comprising a plurality of transistors formed in a polycrystalline semiconductor thin film, wherein the semiconductor device is configured according to any of the foregoing structures. For example, the electronic apparatus may be a liquid crystal display apparatus, in which case the semiconductor device includes transistors for operating respective pixels on a liquid crystal panel of the liquid crystal display apparatus, and transistors forming part of a peripheral driver circuit. The semiconductor device is attached to the liquid crystal display panel overlapping each other.